Structural and magnetic properties of L10-FePd/MgO films on GaAs and InP latticemismatched substrates
M. Kohda, S. Iimori, R. Ohsugi, H. Naganuma, T. Miyazaki, Y. Ando, and J. Nitta

Citation: Appl. Phys. Lett. 102, 102411 (2013);
View online: https://doi.org/10.1063/1.4795443
View Table of Contents: http://aip.scitation.org/toc/apl/102/10
Published by the American Institute of Physics

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APPLIED PHYSICS LETTERS 102, 102411 (2013)

Structural and magnetic properties of L10-FePd/MgO films on GaAs and
InP lattice mismatched substrates
M. Kohda,1,2 S. Iimori,1 R. Ohsugi,1 H. Naganuma,3 T. Miyazaki,4 Y. Ando,3 and J. Nitta1
1

Department of Materials Science, Tohoku University, Sendai 980-8579, Japan
PRESTO, Japan Science and Technology Agency, Kawaguchi 332-0012, Japan3
Departoment of Applied Physics, Tohoku University, Sendai 980-8579, Japan
4
Department of Instrumental Analysis, Tohoku University, Sendai 980-8579, Japan
2(Received 30 January 2013; accepted 28 February 2013; published online 13 March 2013)
Structural and magnetic properties of epitaxial L10-FePd/MgO films on GaAs and InP latticemismatched substrates are investigated at different MgO and FePd growth temperatures. Whilec-axis lattice constants of MgO and FePd show similar values on both substrates, the remanent
magnetization becomes larger on GaAs than that on InP. Since the ratio of FePd (002) tetragonalordered phase and FePd (200) cubic disordered phase follows similar growth temperaturedependence to the remanent magnetization and the long range chemical order parameter, theperpendicular magnetic anisotropy grown on the lattice-mismatched semiconductors is strongly
C 2013 American Institute of Physics.
affected by formation of the disordered phase. V[http://dx.doi.org/10.1063/1.4795443]
In the field of spintronics, a ferromagnet (FM)/semiconductor (SC) hybrid structure has attracted increasing interestfor electrical spin injection and detection1,2 as well as
magneto-optical effect.3,4 Widely studied FM/SC structures,such as Fe, CoFe, and (Ga,Mn)As grown on GaAs
substrate,5–8 usually employ magnetic easy axis along an inplane direction. These structures have demonstrated spinlight emitting diode operation and electrical spin injection/
detection in non-local device geometry.1,2,5–8 However, forfuture spintronic devices combined with magneto-opticalinteraction4 and spin manipulation,9 a perpendicularly magnetized FM/SC structure is indispensable.
Chemically ordered (L10) FePd exhibits large perpendicular magnetic anisotropy of the order of 107 erg/cm3 (Refs.
10 and 11) and high magneto-optical response.12,13 In addition, an L10-FePd thin film can be grown on an MgO layer
which also acts as a tunnel barrier for an efficient spin injection. Hence, the L10-FePd/MgO/semiconductor structure will
be a promising candidate for spin-based electronic and optical devices. For semiconductor materials, GaAs/AlGaAs and
InP/InGaAs heterostructures are widely used for high electron mobility transistors and telecommunication devices
because of small effective mass and direct band gap materials, which will enable us to realize the threshold reduction ofa vertical cavity surface emitting laser8 and a reconfigurable
logic circuit14 based on spin degree of freedom. However,
GaAs and InP substrates exhibit large lattice mismatch to the
L10-FePd and MgO films. In previous studies, while we havegrown L10-FePt/MgO films on GaAs,15,16 the crystallographic origin for degrading the remanent magnetization andthe long range chemical order parameter has not been understood. By comparing structural and magnetic properties of
L10-FePd grown on different lattice-mismatched substrates,
we can clarify the crystallographic origin affecting on theperpendicular anisotropy in L10-based FM/SC hybrid
structures.
In this letter, we sputtered 20 nm L10-FePd/10 nm MgOfilms on GaAs and InP substrates at different MgO and FePd
0003-6951/2013/102(10)/102411/4/$30.00

growth temperatures and investigated the structural and magnetic properties. The growth temperatures were varied fromroom temperature (RT) to 400  C for MgO and from 300  C
to 500  C for L10-FePd, respectively. For both growth conditions, MgO and FePd lattice constants exhibited similar values on GaAs and InP substrates. However, the ratio of
remanent magnetization to saturation magnetization for the
L10-FePd showed higher values on GaAs than that on InP.
By analyzing X-ray diffraction (XRD) patterns, we revealed
that the ratio of FePd (002) tetragonal ordered phase and
FePd (200) cubic disordered phase shows similar growthtemperature dependence to the remanent magnetization as
well as a long range chemical order parameter, which can
explain the crystallographic origin affecting on the perpendicular magnetic anisotropy in an L10-based FM/MgO/SC
system.
L10-FePd and MgO films are grown on GaAs (001) andInP (001) substrates in an ultrahigh vacuum magnetron sputtering system. Prior to the sputtering, a surface oxidized
layer is removed by chemical etching with 35% hydrofluoric
acid (HF) for 1 min followed by 1 min rinse with deionizedwater. The surface roughness after the HF etching is as small
as 0.252 nm measured by atomic force microscopy (AFM).
The etched wafers are immediately transferred to the sputtering chamber to avoid the surface oxidization and areannealed at 400  C for 20 min for surface cleaning. The
growth rate of MgO and L10-FePd films are 0.065 Å/s (Arpressure 0.1 Pa) and 0.114 Å/s (Ar pressure 0.6 Pa), respectively. In order to compare the structural and magnetic
properties of 20 nm L10-FePd/10 nm MgO films on latticemismatched substrates, 10 mm-square wafers of GaAs and
InP are prepared on a single substrate holder and simultaneous sputtering is performed. L10-FePd/MgO films are grownat different MgO and FePd temperatures, TS (MgO) and TS(FePd). For present studies, two sets of samples are prepared:
First set is TS (MgO) ¼ RT, 200  C, 300  C, and 400  C withTS (FePd) ¼ 300  C, and second set is TS (FePd) ¼ 300  C,
400  C, and 500  C with TS (MgO) ¼ 300  C. All FePd films

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Kohda et al.

are covered with 4 nm-thick Ta as a capping layer. The structural, surface, and magnetic characterizations are carried out
by XRD, cross sectional transmission microscopy (TEM),
AFM, and polar magneto-optical Kerr effect (PMOKE).
Since the lattice mismatch between MgO and GaAs
(InP) shows over 25%, which could affect the perpendicularmagnetic anisotropy, we first investigate the MgO growthtemperature dependence at TS (FePd) ¼ 300  C. Figures 1(a)and 1(b) show the XRD results for L10-FePd/MgO films on
GaAs and InP substrates, respectively. In the case of GaAs
substrate, clear MgO (002) peaks are observed at all TS(MgO), which brings about the epitaxial growth of an
L10-FePd as confirmed by FePd (002) fundamental peak as
well as FePd (001) super lattice peak. However, for the InP
substrate, an MgO (002) peak and an L10-FePd phase are
only observed at TS (MgO) ¼ 200  C and 300  C, suggestingthat the MgO crystallinity is affected by different lattice mismatched substrates. In previous study, we have confirmed
the crystallographic relationship between MgO and GaAs as
MgO (001) jj GaAs (001) along the growth direction.15 In
order to figure out the InP case, we carry out the TEM observation for the sample sputtered at TS (MgO) ¼ 300  C.
Figures 2(a) and 2(b) show the cross-sectional TEM imagesof the L10-FePd/MgO/InP structure and Figs. 2(c) and 2(d)exhibit the nano-beam electron diffraction (NBD) patterns
along (1–10) direction in the InP region and in the MgO/InP
interface region, respectively. The high resolution TEM
image shown in Fig. 2(b) reveals the flat interfaces at the
InP/MgO and MgO/L10-FePd, which becomes the advantage
for efficient electrical spin injection. While the lateral crystallite diameter of the MgO is on the order of a few 10 nm
due to the large lattice mismatch between MgO and InP substrate, L10-FePd shows small dispersion of c-axis crystal orientation resulting in the larger crystallite diameter on theorder of a few 100 nm. In Figs. 2(c) and 2(d), we observed

Appl. Phys. Lett. 102, 102411 (2013)

the ordered spot patterns and, by comparing dashed yellow
and blue lines in the NBD patterns, the reflections from the
InP and MgO films are identified as MgO (001) jj InP (001)crystallographic relationship. As a result, the MgO (001)film shows the cube-on-cube epitaxial relationship also for
the InP (001) substrate. Figure 1(c) shows the c-axis lattice
constants of the MgO and FePd films with different TS(MgO) evaluated from XRD results. While the MgO lattice
constant is slightly changed between 0.421 and 0.418 nm,
the similar lattice constant is obtained for the MgO filmgrown on GaAs and InP. The c-axis lattice constants of the
L10-FePd on GaAs and InP become smaller values than bulk
FePd (0.372 nm), indicating a tensile strain induced by the
MgO film. Although the MgO lattice constant shows weektemperature dependence, there is almost no substrate difference both for the MgO and L10-FePd lattice constants.
For further optimizing the structural properties of the
L10-FePd, we next investigate the FePd growth temperaturedependence. We set TS (MgO) ¼ 300  C because of the clear
observation of MgO (002) peak both on GaAs and InP substrates. Figures 1(d)–1(f) show XRD results and evaluatedc-axis lattice constants. For both GaAs and InP substrates,
FePd (002) fundamental peak as well as FePd (001) superlattice peak are observed at all TS (FePd) due to optimized
MgO crystallinity. Since the L10-FePd lattice constants arealmost same between GaAs and InP substrates, similar crystallographic properties are obtained in the L10-FePd/MgO
films on GaAs and InP substrates.
In order to investigate magnetic property of the
L10-FePd film, we next carry out a PMOKE measurement byapplying a perpendicular external magnetic field to the film
plane. Figures 3(a)–3(d) show the out-of-plane magnetic
field dependence of normalized PMOKE signals for the
L10-FePd on GaAs and InP substrates at different MgO
and FePd growth temperatures. The observed hysteresis of

FIG. 1. XRD patterns and lattice constants of MgO and FePd in different growth temperature conditions. XRD patterns of different MgO growth temperatureon (a) GaAs and (b) InP and (c) evaluated c-axis lattice constant from XRD measurement. XRD patterns of different FePd growth temperature on (d) GaAs
and (e) InP and (f) c-axis lattice constant.

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Appl. Phys. Lett. 102, 102411 (2013)

FIG. 3. Magnetic field dependence of normalized PMOKE signals for different MgO temperature on (a) GaAs and (b) InP and for different FePd
temperature on (c) GaAs and (d) InP. The ratio of remanent magnetization
to saturation magnetization (Mr/Ms) as a function of (e) MgO and (f) FePd
growth temperatures.
FIG. 2. (a) Cross-sectional TEM image. (b) High resolution TEM image.
The selected area corresponds to the dotted square region in (a). Nano beamdiffraction patterns taken from (c) InP layer and from (d) MgO/InP
interface.

L10-FePd on GaAs shows better squareness and larger coercivity than that on InP. As shown in Figs. 3(e) and 3(f), theratio of remanent magnetization to saturation magnetization(Mr/Ms) exhibits clear temperature dependence both for
GaAs and InP substrates. Despite the similar lattice constantsof the L10-FePd and MgO films on GaAs and InP substrates,
much higher Mr/Ms is obtained for the L10-FePd on GaAs
than that on InP. In order to explain the difference of Mr/Ms,
we first focus on the FePd (001) super lattice peak in thegrowth condition of TS (MgO) ¼ 300  C and TS (FePd)¼ 500  C. As shown in Fig. 4(a), full-width at half maximum(FWHM) of the FePd (001) on InP shows 1.187 , while the
FWHM on GaAs becomes narrower value of 0.897 . Similar
FWHM dependence is also observed in different MgOgrowth temperatures. Since both peaks are well fitted by asingle Lorentz function (Black lines in Fig. 4(a)), such a difference indicates that the distortion of the MgO c-axis fromthe perpendicular direction depends on the lattice mismatched substrates, which influences on the structural andmagnetic properties of L10-FePt. To further investigate the
MgO lattice distorted effect to the L10-FePt structural properties, we next compared FePd (002) fundamental peaks on
GaAs and InP as shown in Figs. 4(b) and 4(c). The dottedand dashed lines correspond to the calculated peak positionof FePd (002) and FePd (200). Unlike the FePd (001) peaks
in Fig. 4(a), two different peaks are superimposed near the

FIG. 4. (a) FePt (001) super lattice peaks grown on GaAs and InP at
Ts(MgO) ¼ 300  C. Black lines are the fitting with single Lorentz function.
FePd (002) fundamental peaks for different growth temperatures on (b)GaAs and (c) InP.

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Kohda et al.

FIG. 5. The ratio of FePd (002) tetragonal ordered phase and FePd(002) þ FePd (200) phases as a function of (a) FePd and (b) MgO growthtemperatures. The long range chemical order parameter S as a function of(c) FePd and (d) MgO growth temperatures.

FePd (002) fundamental peak position. The second peak islocated at lower angle, indicating the coexistence of FePd(002) tetragonal ordered phase and FePd (200) cubic disordered phase. In order to estimate the ratio of these two phases,
we extract the relative areas of FePd (002) and FePd (200)diffraction patterns by fitting two Lorentz functions. Figures
5(a) and 5(b) show the ratio between the FePd (002) orderedphase and the FePd (002) þ FePd(200) phases as a function of
FePd and MgO growth temperatures. In all samples, the ratioof the FePd (002) tetragonal ordered phase is higher for the
L10-FePd on GaAs than that on InP. It is noted that the ratioof the FePd (002) tetragonal ordered phase follows similar
temperature dependence to Mr/Ms in Figs. 3(e) and 3(f). Since
the remanent magnetization of the disordered phase is aligned
in an in-plane, the increase of the disordered phase decreases
the remanent magnetization. As a result, the temperature dependence of Mr/Ms is explained by the formation of the FePd(200) cubic disordered phase. Finally, we extracted the longrange chemical order parameter S in Figs. 5(c) and 5(d).
While S  0.6 is constantly obtained for the L10-FePd on
GaAs in different growth temperatures, S for the L10-FePd on
InP reduces to 0.38–0.55. The temperature dependence of S
parameter follows Figs. 5(a) and 5(b), suggesting that the formation of the FePd (200) cubic disordered phase affects bothon the Mr/Ms and the S parameter for L10-FePd grown on
GaAs and InP. Since the cubic disordered phase is also
observed in different L10-based materials grown on GaAs
substrate and reduces the Mr/Ms,16 reduction of the cubic disordered phase would be a key to achieve the larger perpendicular magnetic anisotropy on semiconductor substrates.
Difference of the FePd (200) cubic disordered phaseformation between GaAs and InP substrates might be due tothe lattice periodicity between MgO and GaAs(InP). The inplane lattice length of three GaAs unit cells corresponds toAppl. Phys. Lett. 102, 102411 (2013)

the length scale of four MgO unit cells with 0.44% latticemismatch, which could suppress the c-axis distortion during
the MgO growth. However, an InP unit cell does not holdsuch a relationship to an MgO lattice constant. As can beseen in Fig. 2(a), the misfit dislocations are observed fromthe interface of the FePd/MgO. Such a dislocation of the
FePt film enhances the FePd (200) cubic disordered phaseduring the growth, resulting in the degradation of remanent
magnetization on InP in comparison with GaAs substrate.
In conclusion, we have investigated structural and magnetic properties of the L10-FePd/MgO films on GaAs and
InP lattice mismatched substrates by changing the MgO and
FePd growth temperatures. While the lattice constants of the
MgO and L10-FePd on GaAs and InP shows the similar values, the Mr/Ms ratio exhibits clear temperature dependence
both for GaAs and InP and becomes much larger values for
the L10-FePd on GaAs than that on InP. This temperature dependence is explained by the relative ratio between the FePd(002) tetragonal ordered phase and the FePd (200) cubic disordered phase. Since the temperature variation of long range
chemical order parameter S also follows to the disorderedphase ratio, suppression of the cubic disordered phasebecomes important for the perpendicularly magnetized
FM/SC hybrid structures.
We thank Professor Seki and Professor Takanashi for
PMOKE measurement and useful discussions. This work
was supported in part by Grant for Advanced Industrial
Technology Development from NEDO, by the Japan Scienceand Technology Agency (JST) through its Strategic
International Cooperative Program under the title“Advanced spintronic materials and transport phenomena(ASPIMATT)” and by the Mayekawa Foundation.

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